Backport HeaderActionHook and HeaderDropdownActionHooks to 1.7.
Review Request #4703 — Created Oct. 8, 2013 and submitted
Backport HeaderActionHook and HeaderDropdownActionHooks to 1.7.
Cherry-picks 33ed256007 from master, with the one small modification
whereby we use the old-style dropdown.png instead of ▾ in the
templates. This is for consistency, since that's what 1.7 always
used for the dropdown marker.
Passes all HookTests, and introduces some new ones, which also pass. I was also able to successfully load my TogetherJS extension on 1.7 using this patch.
